1. Field of Invention
This invention relates to tunable duplexers.
2. Prior Art
A duplexer is a device that isolates a receiver signal from a transmitter signal while permitting a receiver and transmitter to share a common antenna. The duplexer must be capable of handling the transmitter power and be able to provide sufficient isolation to prevent receiver desensitization due to coupling of the transmitter signal into the receiver. When the transmit and receive frequencies are different, filters may be used to reduce the transmit signal levels to an acceptable low level at the receiver input.
Nontunable duplexers used in wide bandwidth systems such as CDMA systems use a well-known method of creating the wide pass band bandpass filters by cascading resonant sections coupled to additional sections. The more sections, the wider the pass band bandwidth. For example, for a CDMA system operating in the AMPSs band, the Tx frequency band is 824 to 849 MHz and the Rx frequency band is 869 to 894 MHz. Fixed nontunable duplexers must be designed to pass all channels in the band. This would require a nontunable filter with a pass band bandwidth of greater than 25 MHz to compensate for process and temperature variations. Many cascaded sections are also needed to achieve this bandwidth and steepness in the transition bands.
In the past, duplexers were a fixed design with the frequency bands of the transmit and receive bands predetermined. A tunable duplexer can simplify the design because of the ability to tune to the desired narrowband channel. The need for tunable duplexers existed and the following references reflect the current state of tunable duplexers and tunable filters.
U.S. Pat. No. 6,990,327 issued to Zheng et al entitled “Wideband Monolithic Tunable High-Q Notch Filter for Image Rejection in RF Application”, incorporated herein by reference, describes a tunable notch filter which is contained on a single integrated chip.
U.S. Pat. No. 6,407,649 issued to Tikka et al entitled “Monolithic FBAR Duplexer and Method of Making the Same”, incorporated herein by reference, describes a monolithic bulk acoustic wave (BAW) duplexer. A patterned piezoelectric material is used as the piexolayer for each of the resonators of the duplexer.
U.S. Pat. No. 6,816,714 issued to Toncich, entitled “Antenna Interface Unit”, incorporated herein by reference, describes a ferro-electric tunable duplexer.
U.S. Patent Application Publication 2003/0199286, inventor D du Toit, entitled “Smart Radio Incorporating Parascan Varactors Embodied Within an Intelligent Adaptive RF Front End”, incorporated herein by reference, describes a radio with an RF front end containing at least one tunable duplexer. The radio incorporates Parascan® electrically controlled dielectric varactors embedded within the RF front end. Parascan® is a trademarked tunable dielectric material developed by Paratek Microwave, Inc. The RF front end described in the publication contains a tunable duplexer and two tunable filters.
An article, entitled “A Novel Electronically Tunable Active Duplexer for Wireless Transceiver Applications”, by B. Sundaram and R. Shastry, published June 2006 in Vol. 54, No. 6 of IEEE Transactions on Microwave Theory and Techniques, describes a 4-port duplexer circuit that uses varactors to enable electronic tuning of the frequency at which isolation is desired.
An article, entitled “Adaptive Duplexer Implemented Using Single-Path and Multipath Feedforward Techniques with BST Phase Shifters”, by T. O'Sullivan, R. York, B. Noren, and P. Asbeck, published January 2005 in Vol. 53, No. 1 of IEEE Transactions on Microwave Theory and Techniques, describes a technique to enhance the isolation of a surface acoustic wave duplexer, which reduces the noise levels in the receive band of the system. Feedforward techniques are used to create an adaptive null in the receive band.
An article, entitled “A Varactor Tuned RF Filter”, by A. Brown and G. Rebeiz, published Oct. 29, 1999 as a submission for review as a short paper to the IEEE Transactions on MTT, describes an electrically tunable filter at 1 GHz. The resonators used in the tunable filter are stripline interdigital fingers with varactor diodes at the ends.
U.S. Patent Application Publication 2005/0148312, inventors Toncich et al, entitled “Bandpass Filter with Tunable Resonator”, incorporated herein by reference, describes a tunable bandpass filter comprising of ferroelectric tunable tank circuits.